Contrasting effects of grazing and hay cutting on the spatial and genetic population structure of Veratrum album, an unpalatable, long‐lived, clonal plant species

Abstract

Summary 1 Vegetation change induced by large herbivores is driven by the effects of grazers on populations of individual plant species. Short‐term experimental or demographic studies may be insufficient when investigating the population responses of long‐lived clonal plant species. 2 We therefore examined the effects of grazing on such a plant (Veratrum album) by comparing the spatial and genetic structure of populations in grasslands subject to long‐term grazing or mowing for hay. 3 V. album is a locally dominant species that is avoided by large herbivores due to its toxicity. RAPD‐phenotypes of a subsample of c. 50 shoots, and co‐ordinates and dry weight of all shoots, were determined in a 5 × 10 m plot in each of four meadow and four pasture populations. 4 The breeding system of the genus Veratrum was previously unknown but our experimental finding that cross‐pollinated but not self‐pollinated or unpollinated flowers produced as many seeds as freely pollinated flowers suggested that V. album is a predominantly cross‐pollinating species. 5 Both the spatial and genetic population structure differed markedly between the two grassland types. Clonal expansion of established plants in pastures led to populations consisting of larger shoots that were significantly more aggregated at a small spatial scale. Populations also had a higher proportion of flowering shoots, less seedling recruitment and a lower genotypic diversity in pastures than hay meadows. 6 The differences in population structure appear to be due to hay meadow populations reproducing primarily by seeds, whereas clonal reproduction accounts for half of the population growth in pastures. We suggest that, as livestock selectively avoids V. album shoots, grazing indirectly promotes plant growth, which results in an enhanced vegetative reproduction as well as a higher seed production. Experimental studies are, however, needed to determine why and how grazing adversely affects seedling recruitment. 7 Detailed information on population level responses of unpalatable dominant plant species, such as provided by the present study, may help us understand and predict vegetation change in response to changing levels of herbivory.